In the field of subsea hydrocarbon wells, it is known to distinguish between conventional or vertical Xmas trees (XT) on one hand and horizontal trees on the other hand. Traditionally, a vertical XT has a production flow passage extending through a vertical bore of the tree. Installed in the vertical production flow is a production master valve (PMV) adapted to open and close the passage. The PMV is hydraulically controlled and provided with a fail close mechanism. If the hydraulic pressure drops, the PMV will close. Another characteristic of vertical XT's is that the tubing hanger (TH) from which a production tubing suspends and extends into the well, is arranged in the well head below the XT.
Common for both vertical and horizontal XT's is a horizontal flow passage that branches out from the vertical bore. However, while the vertical tree has the PMV in the vertical bore the horizontal XT's have the PMV arranged in the horizontal flow passage. Another difference from the vertical tree is that the tubing hanger is installed within the vertical bore of the tree, instead of below the tree, such as in a wellhead or a tubing head spool.
A traditional consequence of the features of the vertical XT is that the operator can remove the tree without removing the TH. However he cannot remove the TH without removing the tree. Contrary to this, the traditional consequence of the features of the horizontal XT is that the operator cannot remove the tree without removing the TH, but can remove the tubing hanger without removing the tree.
There are various solutions which address these drawbacks. Patent publication EP 0611874 B1 describes a subsea wellhead assembly adapted in such way that the operator can remove the tubing without removing the Xmas tree, and remove the Xmas tree without removing the tubing. The tubing is suspended in a lower tubing hanger arranged in the wellhead housing. The tubing can be removed through the tree and a blowout preventer (BOP). When removing the tree, a plug is set in the lower tubing hanger within the wellhead housing. During production, the production flow is guided from the lower tubing hanger up to an upper tubing hanger that exhibits a lateral port that registers with an outlet port in the tree. The upper tubing hanger (referred to as a false tubing hanger) and a tree cap installed in the tree constitute two barriers.
US patent application publication US2007246220 also describes a subsea wellhead assembly having a tubing hanger in the wellhead housing. A spool, being part of a XT assembly, lands on the wellhead. A tree cap extends into the bore of the spool. A lateral passage of the tree cap aligns with a lateral port of the spool, through which fluid flows during production.
Patent publication U.S. Pat. No. 7,677,320 describes the use of an electrical pump lowered into the tubing when the downhole pressure becomes low. The tubing comprises a downhole safety valve which is closed during installation of the pump. It has a tubing hanger arranged in the XT. The tubing hanger has a port aligned with a horizontal outlet passage in the XT.